Hitherto, aluminium plates have been widely used as supports for planographic printing. The surface of aluminium plates is ordinarily grained for the purpose of improving an adhesive property to the light-sensitive layer provided thereon, and of improving a water retentive property of nonimage parts (areas where the surface of the support is exposed, which receive dampening water used for printing, and which repels oily inks) of planographic printing plates produced with them. This graining processing is called graining, and is an essential step for preparation of a support for planographic printing. The graining processes known include mechanical graining processes such as ball graining, wire graining, brush graining, blast graining, etc., electrolytic graining processes which comprise carrying out electrochemical etching in an acid or neutral aqueous solution, and chemical graining processes which comprise chemically etching with acid or alkali using specific aluminium alloy materials. In Heiban Seihan Insatsu no Kiso, published by Insatsu Jiho Co., pages 35-37, by Kenichi Sugiyama, Dec. 1, 1965, a general explanation of graining processes is described, and a process which comprises carrying out mechanical graining with using an abrasive slurry is specifically described. In Insatsu Zasshi (Japan Printer), July 1963, pages 2-4 in an article entitled "General Statement of Alumite Planographic Plate", by Sano, a process for producing planographic printing plates which comprises electrolytically graining by an alternating electric current in a hydrochloric acid or nitric acid solution, and thereafter carrying out anodic oxidation, is described. Japanese Patent Publication No. 42284/79 discloses a process for producing planographic printing plates wherein an aluminium plate containing from 1.6 to 2.5% of manganese is subjected to chemical graining with alkali.
Complex graining processes suitably combining these known graining processes are also known. For example, U.S. Pat. No. 2,344,510 discloses a process for producing a support for planographic printing which comprises carrying out mechanical graining and then electrochemical graining with hydrochloric acid etc., to form superimposed complex grains on the surface, and thereafter carrying out anodic oxidation. Japanese Patent Publication No. 16918/82 and British Patent B.P. No. 2,047,274 have disclosed a process for producing a support for planographic printing which comprises carrying out mechanical graining, chemical etching with acid or alkali and electrochemical etching in turn, and thereafter carrying out anodic oxidation. Further, British Pat. No. 2,118,575 discloses a process for producing a support for planographic printing which comprises carrying out mechanical graining, chemical etching and electrochemical graining in an acid electrolytic solution by an alternating wave-form electric current to form a grain structure consisting of plateaus and pits on the aluminium surface.
It has been found that the shape of fine unevenness observed in the case of magnifying the surface of the grained aluminium support from 2000 to 5000 times has the following very great influences upon printing performance when used as a support for planographic printing:
(1) It causes firm adhesion between the image layer and the support to improve printing durability; PA1 (2) It causes firm adhesion to the support throughout the development step for fine halftone images of from 1 to 5% or so to the printing step to improve tone reproduction; PA1 (3) It causes firm adhesion of a hydrophilic coating layer (for example, gum arabic) to the nonimage parts to improve hydrophilic property; and PA1 (4) It causes improvement of the water retentive properties of the nonimage parts.
As a result of observation and extensive studies with respect to this fine unevenness structure, the following has been discovered.
(a) Generally, the grained surface has a coarse structure (primary structure) and a fine structure (secondary structure), and fine unevenness having the above described effects originates in the secondary structure.
(b) In carrying out mechanical graining, it is possible to control the secondary structure by selecting granularity of the abrasive used and by carefully determining processing conditions.
This plan has been disclosed in Japanese Patent Publication Nos. 42282/79 and 42283/79, as a process for producing aluminium printing plates for offset which comprises carrying out mechanical graining with two kinds of abrasives, each having a different grain size, by a wet blast method (liquid honing method).
However, since abrasives and cutting dusts of aluminium, etc., remain on the mechanically grained surface, there are generally problems of blackening of external appearance and contamination during printing caused by these residual materials, especially in case of producing a printing plate by carrying out anodic oxidation.
(c) In order to solve the above described problems, it is effective to use a process which comprises carrying out chemical etching with acid or alkali, as described in U.S. Pat. No. 3,834,998 or a process which comprises carrying out electrochemical etching in phosphoric acid and thereafter carrying out anodic oxidation as described in U.S. Pat. No. 3,929,591. However, a new problem thereby arises, namely, the surface character obtained by graining deteriorates, resulting in deterioration of water retention and printing durability, because the secondary structure tends to disappear in the etching step.
(d) In the case of electrochemical graining, formation of superimposed structure is similarly observed. For example, a report supporting this fact can be found in Sumitomo Light Metal Giho, Vol. 15, No. 2, April 1974, in an article entitled "Process for the Production of Electrolytically Etched and Anodized Strip".
According to the research of the present inventors, a quantity of electricity of from 1000 to 1500 coulombs/dm.sup.2 is required for obtaining an average surface roughness (Ra=0.6) of PS plates (Presensitized plate) conventionally available in the market by graining in an electrolytic solution containing hydrochloric acid as a main component by a commercial alternating electric current. Since the whole surface just after electrolysis is covered with insoluble residues, called smut, which are generally difficult to remove by washing with water or cleaning by a brush, the surface is necessarily etched with alkali, as described in Japanese Patent Publication No. 28123/73. As the result, though the secondary structure is present on the surface which is carefully subjected to slight etching, it disappears when the etching even slightly exceeds an optimum extent, and, consequently, there is a fault that water retention and hydrophilic property deteriorate similarly to (c).
(e) Many attempts of adding various additives to the hydrochloric acid, by which formation of coarse primary structure is prevented to accelerate formation of a fine secondary structure, have been proposed. It has been known hitherto to add additives, for example, additives such as nitric acid, chromic acid or hydrofluoric acid (Japanese Patent Publication No. 28123/73), additives such as amines, aldehydes or nonionic surfactants (U.S. Pat. No. 3,755,116), boric acid (French Pat. No. 2,110,257 and U.S. Pat. No. 3,980,539) and phosphoric acid (U.S. Pat. No. 3,887,447), etc.
Particularly, in U.S. Pat. No. 3,887,447, it has been described that a factor obstructing uniform formation of fine pits is sulfuric ion present in the electrolytic solution as impurities in a very small amount, and that uneven coarse pits are formed when from 10 to 15 ppm of sulfuric acid ion are present, and it has been proposed to add phosphoric acid in order to improve it.
(f) The present inventors have gained confidence that an electrolytic solution containing nitric acid as a main component is advantageous for forming uniformly and effectively a fine secondary structure on the surface, as compared with hydrochloric acid. This reason is that the amount of smut on the surface which is electrolytically grained with an electrolytic solution containing nitric acid as a main component is smaller than that in case of using hydrochloric acid and, therefore, the smut can be removed by comparatively mild post-processing. Thus, it has an advantage that grains obtained by electrolytic graining are hardly destroyed.
However, since a grain structure obtained with an electrolytic solution containing only nitric acid by a commercial electric current has an irregular uneven state containing huge pits, there is a fault that a uniform surface structure having a desired surface roughness necessary for the printing plate can not be obtained.
(g) Further, Japanese Patent Publication No. 764/65 has disclosed a process for forming a fine grain structure which comprises adding protective colloid containing lignin as a main component, aromatic aldehyde or aromatic ketone to nitric acid.
However, according to results of examinations by the present inventors, the effect thereof is still insufficient to completely attain the desired results.